Compounds of copper, chromium and arsenic as wood preservatives

ABSTRACT

The content of arsenic in conventional MeO-CrO3-As2O5 wood preservatives is partially replaced by phosphoric or sulfuric acid.

United States Patent Hager Feb. 11, 1975 COMPOUNDS OF COPPER, CHROMIUM AND ARSENIC AS WOOD PRESERVATIVES [56] References Cited [75] Inventor: Bror Olof Huger, Djursholm, FOREIGN PATENTS OR APPLICATIONS Swede" 27,826 12/1955 Finland 102,457 ll/l937 Australia 424/l37 [73] Ass'gnee' Stockholm l66,4l3 H1956 Australia 424/137 [22] Filed: June 1971 Primary Examiner-Vincent D. Turner [21] APPL 153,412 Attorney, Agent, or Firm-Pierce, Schefi'ler & Parker 0 Related U.S. Appllcation Data I 57] ABSTRACT [63] Contmuanon of Ser. No. 733,825, June 3. I968, abandoned, The content of arsemc 1n convenuonal MeO-- Cro -AS 0 wood preservatives is partially replaced 52 us. (:1. 424/128, 424/137 by phosphoric or sulfuric acid- Int. 3 N D 58 Field of Search 424/137, 128, 141 'awmgs COMPOUNDS OF COPPER, CHROMIUM AND ARSENIC AS WOOD PRESERVATIVES This application is a continuation of my parent patent application Ser. No. 733,825 filed June 3, 1968, which parent application is now abandoned.

This invention relates to the wood preservation art, and is concerned with the provision of improved preservative compositions.

Aqueous solutions containing compounds of copper and chromium have long been used as wood preservatives. The copper compound usually has been copper sulphate, and the conserving effect of the preservative has mainly been based on the copper radical in this compound. The chromium compound has been a water-soluble chromate having the property of protecting ferrous metal impregnating apparatus against corrosion from the copper compound. Furthermore the chromate was observed to have the uniquely important property of fixing the copper compound in situ in the wood in an insoluble condition so that the copper could not be leached when the wood later was exposed to water or moisture. This fixation depended on the reduction of the chromate in contact with the wood, chromium radical hereby being converted from the sixvalent negative, to a three-valent, positive, radical. Thus the chromium in the preservative is present as a negative, strong acid radical (ion) (i.e., chromic acid). As an acid it acts as a dissolving agent with respect to the different compounds in the preservative. The dissolving effect of the chromium compound ceases completely upon its reduction as noted above.

To this effect will be added the precipitating effect of the formed three-valent (basic) chromium radical. The same can neutralize other dissolving acid radicals such, for example, as sulphuric acid. In other cases the positive chromium can form difficultly soluble compounds. This happens when the said preservatives are combined with arsenic compounds, e.g., with arsenic acid or arsenates. When the positive three-valent chromium radical is formed in the wood the arsenic is precipitated wholly or partly as very difficultly soluble chromium arsenates.

A preservative (developed by applicant) which has been used to a large extent in different countries is based on these experiences. The dissolving effect of the chromium and arsenic acids has here been rationally utilized for dissolving the copper compound in the preservative, with very good fixation of the ingredients of the preservative in situ having been realized. This has occurred without forming injurious, soluble, nonconserving products in the wood, for instance, sodium sulphate. This perservative is described in Swedish Pat. No. 139,177 and 143,080 and in US. Pat. Nos. 2,432,007 and 2,565,175. In the last-mentioned patent the claims were directed to a method for manufacturing the preservative but in the specification the limits for the composition of the preservative are given. The preservative is also described in various foreign patents including Finnish Patents 25,857 published in March l949-- and 27,826, both with priority from 1946. In the latter of these Finnish patents the preservative was described with closer limits, the composition of the preservative being stated to be:

(1.5-2.0) CrO 0.752.0) MeO AS205 where Me represents a two-valent metal capable of following formula has been given showing the most suitable composition of the preservative:

1.25 M30 CF03 A5205 If copper (Cu) is introduced where Me is written the exact composition is received of the above said preservative used in practice to a large extent in many different countries.

During the years this preservative has been used many experiences and investigations have given impetus for further development. A first question hereby has been the conserving effect of the copper and the arsenic acid radicals. Gradually it has become apparent that the copper radical has been more active than the arsenic acid which latter only is given a complementary preserving effect. For this purpose the arsenic acid has not been needed in the amount it heretofore had been used. Other circumstances have had an effect in the same direction. The precipitated arsenic compounds undergo a slow hydrolysis-which phenomenon is not discoverable through short leaching testswith the consequence that a part of the arsenic acid'is separated and leached out. The compound remaining after a long leaching thus contains definitely less arsenic had than the original preservative.

Furthermore, questions of poisoning demand lowest possible content of arsenic. As an example it may here be pointed out that by burning wood which had been treated with arsenic compound the amount of arsenic in the ashes can be alarmingly high.

A still further reason has been that a high amount of arsenic in the preservative has been a source of very serious damage to sleepers in whichas rather common todayweak electric current for signals have been running.

All these circumstances show that the preservative to a higher degree than before ought to be based on copper as conserving agent, and that the amount of arsenic acid has to be reduced as much as possible. Different investigations have shown that the copper content in the impregnating solution normally for full-cell treatment of, for instance, Swedish pine may be kept around 0.3 per cent. If the content of arsenic pentoxide (content of arsenic acid) reaches the same extent as does the copper it is surely enough. It is not excluded that the content of arsenic pentoxide can be reduced to half this amount but hardly below this limit if it shall be possible to make use of the special conserving effect of arsenic. This means that the amount of arsenic pentoxide in relation to the amount of copper is reduced to 1/3 or [/6 of the amount used in applicants earlier preservative. A variant of the preservative composition according to this invention, containing no arsenic, has been worked out for use in cases when it is desirable to avoid arsenic as it can be in preservatives for instance for sleepers.

With this the proportions between copper and arsenic pentoxide in the preservative is made clear. Further the preservative must of course be so composed that it can be dissolved in water; it must have a corrosion action on iron or metals; and, after the treatment, it must be precipitated in the wood. The rules for the dissolution is rather simple. One mol CuO is dissolved by 2 mols CrO or by one mol H Even the arsenic acid has some dissolving effect, but it is not of crucial importance in preservatives according to the present invention which now contain compositions comparatively low in amounts of arsenic acid. For dissolving copper more arsenic is necessary than corresponding to monocopperarsenate.

The amount of chromic acid needed for the fixation given: the first figure relates to the amount of arsenic pentoxide, both in hundreds of a per cent. 3/0" thus means that the solution contained 0.30 per cent copper (Cu) and no arsenic pentoxide. 30/45 means 0.30

and the results are given in figures meaning the remaining amount in per cent of the different ingredients. As headingv in a asli a sa ssrws fisusshasbses of the copper radical alone, or of copper and arsenic 5 per cent copper and 0.45 per cent arsenic pentoxide. acid, has been investigated by leaching tests on wood The first vertical line below each heading means per treated with solutions of different compositions. cent remaining amount of copper in the wood after Hereby analyses have been carried out on leaching leaching; the other line means per cent remaining water and on the leached wood in order to ascertain the amount of arsenic pentoxide in the wood after leachresistance to leaching of the different ingredients in the ing. Both lines are given values for increasing chromic preservative. In order to investigate the fixation of the acid content.

/0 l5/7,5 l5/l5 l5/22,5

copper radical a certain amount of copper has been in- From the reported results of these tests it is clear that troduced into a solutior 1 together with different acornparatively good fixation is obtained when the amounts of chromic acid. As a certain amount of chrochromic acid amounts to 25 P cent of the Come! i a id was id b di lv l a certain needed for the formation of bichromate. Twice that amount of copper oxide (in this experiment carbonate amounti.e., 50 P centgives a margin of Securityhas been used as it reacts faster, and the carbon dioxide The corrosion tests show that it is not possible to reevaporates from the acid solutions), a smaller amount duce the amount of the chromic acid drastically withof chromic acid has been used than corresponding to out taking serious risks for damages to apparatus; howbicarbonate of copper (1 mol CuO to 2 mols CrO the ever, if a part of the sulphuric acid is replaced by anexcess of copper having been dissolved in sulphuric 4 other acid this problem can be ketp under control. acid. A surplus of sulphuric acid has been avoided as Here phosphoric acid has proved to be specially suitthis could adversely influence the ensuing fixation. able. If all of the sulphuric acid is replaced by phos- In the experiments reported hereinbelow the chromic phoric acid in the different tests no corrosion attack ocid c t t h v ted t 25, 50 75 d 100 er curs in the above described cases; the same is true if the cent of the amount corresponding to the formation of amount of chromic acid is reduced to P cent of soluble bichromate. 1n the first case 75 per cent of the th m n rr p n ing to ichr m If half the copper has been dissolved by sulphuric acid; in the secamount of sulphuric acid is replaced by phosphoric ond case in the third case 25; while in the fourth acid a content of chromic acid amounting to 50 per case all of the copper was (or, could be) dissolved by 50 cent of the content corresponding to copper bichrothe amount of chromic acid present in the composition. mate will give a non-corrosive solution. It is here to be In order to make it clear the ingredients of these differse e that 0116 H101 sulphuric acid (which dissolves ent solutions can be stated the following way: 1 mol copper oxide) in diluted solutions in equivalen- CuO 0.5 CrO 0.75 H 80 tin dissolving effect-to one mol or some more of CuO 1.0 CrO 0.50 H SO phosphorous pentoxide. Somewhat more phosphoric CuO 1.5 CrO 0.25 H SO acid than that corresponding to mono-copper phos- CuO 2.0 CrO 0.00 H SO phate thus is necessary for obtaining dissolution.

In these reported experiments not only the amounts Sulphuric acid may be replaced by other acids but of chromic acid and sulphruc acid have been varied in the possibilitiles here are very limited. Nitric acid may this way but also different concentrations of the solube used but more corrosion will thereby be obtained. tions have been tested. The solutions have, further, If hydrochloric acid is used an increased risk of corrobeen tested after addition of arsenic pentoxide in consion is of course obtained. The introduction of phostents amounting to the half and the same amount as the phoric acid in the preservative composition causes copper content according to what above have been some changes in the fixation of the ingredients. Thus said. In this way a number of solutions have been tested the fixation of the copper radical is increased while the fixation of the arsenic acid will be decreased to some degree. If half the content of the sulphuric acid is repktcedbyphosphoric acid a good fixation of both copperand arsenicacid radicals is obtained. These facts acid and phosphoric acid have been added to the premay be seen from the results of the following figures seservative it has been made clear that a less surplus of cured from some leaching tests: acid can be tolerated.

Remaining amount of original ingredients after leaching if the below given acids are used:

Mixture of The composition Amount Sulphuric acid Phosphoric sulphuric and chromic of the solution acid acid phosphoric acid It is thus advisable to use a chromic acid content in EXperimeflls with iljfleased mount of P P P P acid the preservative amounting at least to 25 per cent of preservauve the": Sulphur: the amount corresponding to the formation of copper Amount bichromate. More than 50 per cent is not necessary for M299 /39 3o/30 s i having a good fixation if the copper amount not dis- 25 i 32'; if}? 32-3 28-; solved by the chromic acid is dissolved to the same and 50 equal parts by sulphunc acid and phosphoric acid. gg 30/30 +04 Under these circumstances (1) a good dissolution of 82 5 3 3 i 4:3 the copper radical, (2) a non-corrosive solution and 30 (3) the best possible fixation of the ingredients sof the 92.5 64.6

preservative are obtained. As stated above, the content of arsenic pentoxide has to amount to the same or half the copper content.

If the arsenic acid is completely omitted and the pre- Above S presupposed that the smallest possible servative is exclusively based on the copper radical the 35 amount of radicals which can form soluble compounds preservative is made in similar way. An amount of in the wood after treatment has to be used in the prechromic acid corresponding to from 25 to 50 per cent servative. As radicals composing sodium, potassium of the formation of bichromate may be used. It is desirand ammonium easily form soluble salts, these radicals able to increase the amount of the dissolving acids for 40 have been omitted. The sulphuric acid content has ensuring the stability of the solution. Phosphoric acid been t it is not Pr o a higher degree than or a i t e f phosphoric and sulphuric acid, is used that which is precipitated in the wood. in the form of and in both cases safety against corrosion is obtained. basic salts. The phosphoric acid is fixed very effectively the sulphuric acid has here no function in increasing in the amounts used here. the fixation as when arsenic acid is present. For the fix- In order further to make the composition of the preation of the copper radical, on the contrary, phosphoric servative clear some additional examples will now be acid is better. The sulphuric acid may however be ingiven. For one mol CuO in the preservative one mol cluded in order to make the preservative easier to dis- CrO is introduced, which dissolves half a mol CuO. lv The resulting. half part of the copper oxide has as said Experiments with an increased amount of sulphuric acid in the preservative there is no phosphoric acid present.

Tests have also been carried out in orderto establish above to be dissolved by equal parts of sulphuric and the influence an excess of acids in the preservative will phosphoric acid that is by 14 mol H SO and 7 P 0 If have on the fixation of different ingredients. From the the t t f ar nic pentoxide amounts to the same following leaching tests where a pl of ph c content as copper this means an addition of some more than /4 mol As O The composition of the preservative will thus be the following:

If the different terms in the formula are multiplied by 4 the composition can be written in the following way:

2 CuCr O CuSO Cu(H PO AS205 This is valid for diluted solutions. When the preservalives are made in the form of solid salts and further have to be resistant to some heating during manufacture, the amounts of chromic acid (and eventually of the sulphuric-phosphoric acids) have to be increased slightly. When the content of arsenic pentoxide is the same as the copper content (a) the chromic acid amount ought to be increased from 50 to 52.5 per cent of the amount corresponding to copper bichromate. If the amount of arsenic pentoxide is half this value, (b) the amount of chromic acid ought to be increased from 50 to 60 per cent. In the cases when the arsenic pentoxide is omitted (c) the chromic acid amount ought to be increased to 70 per cent. If in the last case only phosphoric acid is used (and no sulphuric acid) and if so much phosphoric acid is introduced that 50 per cent of the copper corresponds to monophosphate (d) 85 per cent chromic acid ought to be introduced; if the amount of phosphoric acid is increased to twice this amount (e) the amount of chromic acid ought to be 65 per cent.

Above described preservatives can be introduced from the following ingredients:

a. 564 parts by weight CuO 589 parts by weight CuSO 5 aq 991 parts by weight CrO 600 parts by weight A5 546 parts by weight H PO.,, 85 per cent b. 564 parts by weight CuO 589 parts by weight CuSO 5 aq l 133 parts by weight CrO 300 parts by weight As O 546 parts by weight H PO 85 per cent 0. 564 parts by weight CuO 589 parts by weight CuSO 5 aq 1322 parts by weight CrO 546 parts by weight H PO 85 per cent (1. 752 parts by weight CuO 1604 parts by weight CrO 1092 parts by weight H PO 85 per cent e. 752 parts by weight (uO 1227 parts by weight CrO, 2184 parts by weight H,PO 85 per cent We claim: 1. A preservative for wood with the following approximate composition 564 parts by weight CuO CuSO, 5 H 0 589 parts by weight CrO 991 parts by Weight A5 0, 600 pans by weight H PO 85 546 pans by weight.

2. A preservative for Wood having the composition in molar relationship CuO CrO A AS205 A P 0 )4; H 

1. A PRESERVATIVE FOR WOOD WITH THE FOLLOWING APPROXIMATE COMPOSITION
 2. A preservative for wood having the composition in molar relationship CuO + CrO3 + 1/4 As2O5 + 1/4 P2O5 + 1/4 H2SO4.
 3. A preservative for wood having the following composition: 564 parts by weight CuO 589 parts by weight CuSO4 . 5 aq 1133 parts by weight CrO3 300 parts by weight As2O5 546 parts by weight H3PO4, 85 per cent. 